Marrow fibrosis is a pathological condition characterized by abnormal accumulation in the bone marrow of fibroblastoid cells and collagen fibers. It can be idiopathic, though it is most often a feature of a variety of malignancies of the hematopoietic system, and of non-malignant pathologies such as hyperparathyroidism and renal osteodystrophy. In recent years, we have generated a mouse model of marrow fibrosis by expressing a constitutively active receptor for Parathyroid Hormone (PTH) in osteoblasts (PPR*Tg). The analysis of this mouse model has led us to the discovery of a novel bone marrow population constituted by cells that express both mesenchymal and hematopoietic markers. This novel population, which we have also successfully identified in normal mice, appears to be significantly expanded in PPR*Tg. Fibrocytes are an intermediate stage of differentiation into mature mesenchymal cells of bone marrow-derived precursors of the hematopoietic/monocyte lineage. They are typically found at sites of wound healing and pathological fibroses, and they have mixed features of hematopoietic and mesenchymal cells. They contribute to wound healing and to pathological fibroses by secreting matrix and pro-angiogenic factors, and by differentiating into myofibroblasts. They can also differentiate into adipocytes in vivo and in vitro. More recently, fibrocytes have been found in lesions of patients affected by Fibrodysplasia Ossificans Progressiva. Since the novel cells we have recently identified in the bone marrow, similarly to fibrocytes, co- express hematopoietic and mesenchymal markers, we named them fibrocyte-like cells. Of note, to this end presence of fibrocytes in the bone marrow has not been reported in a direct fashion. Our current working hypotheses is that the bone marrow, like wounds and pathological fibroses, is a permissive microenvironment for the differentiation of fibroctye-like cells from hematopoietic precursors, and that these cells may contribute to the marrow fibrosis and/or to bone homeostasis with yet unknown mechanisms. In order to start testing our hypotheses, we propose to investigate the identity of the novel bone marrow cells we have recently discovered, by studying whether they are transplantable and of hematopoietic origin, and whether express, in addition to type I collagen, other matrix proteins and pro-fibrotic/proangiogenic cytokines (Aim I). Moreover, we will test whether these cells differentiate into osteoblasts and/or adipocytes (Aim II). The accomplishment of the experiments described in Aims I and II will likely expand our knowledge on the pathogenesis of bone marrow fibrosis, at least in a context of chronic activation of the PTH receptor.